Rotating components such as gas turbine engine rotors (such as compressor and turbine discs and drums), wheels, crankshafts and drive shafts, which rotate about a centre of rotation, may become unbalanced such that the centre of mass of the rotor does not correspond to the centre of rotation. Once unbalanced, such rotors may then be subject to vibration in use, which is undesirable. An unbalanced rotor may then be balanced using any of several methods.
Conventional gas turbine engines, such as gas turbine engine 10 shown in FIG. 1, comprise a plurality of rotors, including fan 12, compressor rotors 16, 18 and turbine rotors 22, 24, 26. As shown in further detail in FIG. 2, each rotor comprises a disc 17 and a plurality of blades 19 removeably attached at the radially outer rim of the disc 17. Due to manufacturing variation, the blades 19 vary in size and mass distribution relative to one another, and relative to a nominal design. Material may also be moved or removed during use, for example by foreign object damage, which would also alter the size and mass distribution of the blades 19.
In a first prior balancing method, the blades 19 can be distributed around the circumference of the disc 17 in such a way that the variations in their weight can be used to compensate for rotor unbalance. In gas turbine engines, several discs are sometimes joined together to form a compressor drum 13 (see FIG. 2) or turbine drum (not shown). Balancing is then conventionally achieved with the rotors attached to form the drum 13.
However, in order to save weight, some gas turbine engine rotors comprise discs having integrally formed blades, known as “bladed disks” or “blisks”. In some cases, the rotor comprises a ring having integrally formed blades, known as “bladed rings” or “blings”. Since the blades are integrally formed with the disc/ring, the blades cannot be circumferentially distributed to balance the rotor.
In a second prior method, rotors such as blisks can be balanced by the removal of material from specific locations designed into the component known as “balancing lands”, for example on a non-aerodynamic profile, such as the disc. During balancing, material is removed from the balancing land to reduce the weight of the blisk, and thereby balance the blisk or drum. However, the balancing land increases the overall weight of the disc. This can have a significant impact on the rotor as a whole, as still further material may be required in the disc to increase the strength to offset the increased centrifugal force produced by the increased weight in use. Furthermore, once all of the material has been removed from the balance land, no further balancing can take place. The disc would then have to be disposed of. There is therefore a compromise between weight and longevity of the disc in service using this method.
In a third prior method, balancing can also be achieved by adding bolted weights or dome head rivets to individual discs, or to the front and rear surfaces of the drum. However, this method may require access to both sides of the component, which may not be possible where the drum is welded together for instance. Such a method also requires a hole to be made in the surface of the disc, which may weaken it. The bolts used also have a minimum practical size. As a result, relatively large discrete weights are added using the process such that “fine tuning” of the balance state is difficult to achieve.
The present invention seeks to provide a method of balancing a rotor that overcomes some or all of these problems.